feat: add `ProjectionExpr` (#41) #21

Job	Run time
lint-and-test / Test Suite	9m 7s
lint-and-test / Check Package	3m 54s
lint-and-test / Clippy	2m 29s
lint-and-test / Format	12s
lint-and-test / Unused Dependencies	6m 48s
Release	5m 47s
	28m 17s

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